Staphylococcus aureus is a gram-positive bacterium that is a significant cause of disease throughout the world. The organism is ubiquitous, with estimates of almost 40% of humans being colonized on mucosal surfaces. See, Lowy, N Engl J Med 339:520-32 (1988); and McCormick, et al., Annu Rev Microbiol 55:77-104 (2001). The illnesses caused by the organism range from relatively benign infections such as furuncles and soft tissue abscesses, to life-threatening illnesses such as toxic shock syndrome (TSS), sepsis, and infective endocarditis. S. aureus causes diseases primarily through production of a large number of virulence factors, both cell surface-expressed and secreted. One of the major secreted exotoxins is the superantigen TSS toxin-1 (TSST-1). See, Bergdoll, et al., Lancet 1:1017-21 (1981); Bergdoll and Schlievert, Lancet ii:691 (1984); and Schlievert, et al., J Infect Dis 143:509-16 (1981). TSST-1 is the cause of menstrual TSS, a condition typically associated with healthy women who are using certain tampons and who have vaginal colonization with S. aureus. See Schlievert, Lancet 1:1149-50 (1986); Schlievert, et al., J Clin Microbiol, 42:2875-6 (2004). Additionally, TSST-1 is the cause of up to 50% of non-menstrual TSS, with most such cases being associated with upper respiratory tract infections; most of the rest of non-menstrual TSS is associated with the superantigens staphylococcal enterotoxins B and C. Superantigens cause serious human illnesses by causing massive cytokine production, resulting in an acute-onset illness characterized by fever and vomiting and diarrhea (flu-like symptoms), hypotension, a sunburn-like rash, peeling of the skin upon recovery, and a variable multi-organ component. See, Davis, et al., N Eng J Med, 303:1429-35 (1980); Marrack and Kappler, Science, 248:705-11 (1990); and Shands, et al., N Engl J Med 303:1436-42 (1980).